The invention relates to an unloader, more particularly an unloader for controlling the fluid output of a hydraulic pump in a constant pressure type circuit, particularly as used in power hydraulic braking systems for motor vehicles. In such systems the pump output is used to fill hydraulic accumulators with pressurized fluid which is, in turn, used for operational duties such as applying the brakes under the command of the brake pedal. When the accumulators are filled with pressurized fluid the pump delivery must be temporarily halted or bypassed and when topping up of accumulator fluid is required the delivery must be recommenced. A device for this purpose is called the pump unloader of which there are basically three types:
(a) Stopping rotation of the pump, as by a clutch device if engine driven, or a switch if motor driven, the command device being usually a pressure rather than volume sensor.
(b) Shunting the unwanted fluid delivery back to reservoir via a control valve.
(c) Interrupting fluid delivery by closing the pump inlet or suction port.
U.K. Patent No. A-1 300 278 discloses a typical pump arrangement using an unloader of type c (above). It is a dual output pump, but could clearly be adapted as a single output pump. The unloader means is a pressurized pin (19) urging a plunger (18) against a spring to close port-hole (16) and stop fluid entering the piston/cylinder assembly (5), which in turn terminates delivery at fitting (11). An alternative construction is shown in U.K. Patent No. A-1485532 in which a pressurized spool (8) cuts off flow through the center of a cup seal (at 10a).